> I think we could just stick with positivity rates and case numbers to figure that out, like we have been doing

Sticking only with positivity rates is misleading even if one ignores all the costs of lockdowns. As the WHO pointed out on 13 January 2021[1]:

> WHO reminds IVD users that disease prevalence alters the predictive value of test results; as disease prevalence decreases, the risk of false positive increases (2). This means that the probability that a person who has a positive result (SARS-CoV-2 detected) is truly infected with SARS-CoV-2 decreases as prevalence decreases, irrespective of the claimed specificity.

You may find this comment[2] by me and the link to the calculator to understand the impact of varying prevalence keeping false positive and false negative rates constant.

Note also,

> Most PCR assays are indicated as an aid for diagnosis, therefore, health care providers must consider any result in combination with timing of sampling, specimen type, assay specifics, clinical observations, patient history, confirmed status of any contacts, and epidemiological information.

In plain English, that says that with no illness and no contact with people suffering from Covid19 etc, a positive test does not necessarily mean that the person testing positive is infected.

This is all basic Stats but it has been conspicuously ignored for almost year now.

> The cycle threshold (Ct) needed to detect virus is inversely proportional to the patient’s viral load.

So, before the test, pick a Ct, and stick with it instead of keeping on going until you get a positive result. At higher counts, the test might be detecting left over material from a long gone infection.

> Where test results do not correspond with the clinical presentation, a new specimen should be taken and retested using the same or different NAT technology.

So, if you are sick and get a negative, test again to make sure you can rule out SARS-Cov2. But, equivalently, if you are not sick and test positive, also make sure that this is not a false positive.

The latter is what has been globally ignored with one positive test on healthy people is regarded as proof of infection and illness. It is what is being ignored when people are not allowed to travel or work due to a positive test result. Basically, a positive test result, even without symptoms, and even with a subsequent negative, puts a scarlet letter on you which cannot be erased.

The flowchart in Figure 1 in this document[3] might also be useful. Note the first box is labeled "Patient meets the clinical criteria for COVID-19". That's where these diagnostic tests are applicable as proof of infection.

The fact that the test is useful for confirmation of infection conditional on presenting symptoms doesn't mean it is useful for screening an entire population or deciding on how much GDP to destroy because of the simple facts that in that scenario a large portion of positive test results will be false positives and these tests have high false negative rates.

[1]: https://www.who.int/news/item/20-01-2021-who-information-not...

[2]: https://news.ycombinator.com/item?id=25894449

[3]: https://apps.who.int/iris/bitstream/handle/10665/334254/WHO-...